Skip PK returned as None for RETURNING, server side default

Skip PK returned as None for RETURNING, server side default

Fixed regression where the ORM exception that is to be raised when an
INSERT silently fails to actually insert a row (such as from a trigger)
would not be reached, due to a runtime exception raised ahead of time due
to the missing primary key value, thus raising an uninformative exception
rather than the correct one. For 1.4 and above, a new ``FlushError`` is
added for this case that's raised earlier than the previous "null identity"
exception was for 1.3, as a situation where the number of rows actually
INSERTed does not match what was expected is a more critical situation in
1.4 as it prevents batching of multiple objects from working correctly.
This is separate from the case where a newly fetched primary key is
fetched as NULL, which continues to raise the existing "null identity"
exception.

Fixes: #7594
Change-Id: Ie8e181e3472f09f389cca757c5e58e61b15c7d79